The present invention relates to an improved hydrothermal process for producing a magnetoplumbitic ferrite powder that can be advantageously used as a magnetic material for high-density recording.
Conventional magnetic recording depends on magnetization which is induced within, and parallel to, the plane of a recording medium. This system is not particularly suitable for high-density recording since its recording density is limited by the demagnetizing field which increases as the degree of recording density in the medium increases. An alternative recording method which is generally referred to as "perpendicular magnetization recording" has been proposed and active efforts are being made towards commercializing this new method.
According to the perpendicular magnetization recording method wherein magnetization is induced in a direction perpendicular to the surface of the recording layer, any two adjacent microscopic magnets in the medium have dissimilar poles at the interface, and this causes a smaller demagnetizating field and ensures the retention of a larger remanence. At shorter wavelengths, a stronger magnetization is induced since the demagnetizing field is reduced and adjacent magnets having opposite mating poles attract each other. Because of these features, the perpendicular magnetization recording system is inherently adapted to high-density recording.
The medium used in the perpendicular magnetization recording system must have an axis of easy magnetization that is oriented in a direction perpendicular to the surface of the medium, and a sputtered Co-Cr film has been proposed for use as such medium. However, this Co-Cr film involves complicated procedures in its fabrication and is not cost effective because of the need to use expensive materials.
A coated type recording medium which is prepared by applying a powder of magnetoplumbitic ferrite powder to a base film is gaining acceptance as an alternative to the sputtered Co-Cr film and considerable effort is being made to commercialize this type of recording medium. There are several advantages inherent in the new recording medium wherein a powder of magnetoplumbitic ferrite powder is coated onto a base film: first, the technique of coating a magnetic layer onto a base film has been practiced for many years in the manufacture of other types of magnetic recording medium and the accumulated experience can be readily applied to the manufacture of perpendicular magnetization recording media; secondly, the existing manufacturing facilities can be used so as to avoid any need to make a large initial investment; thirdly, the coating technique is more efficient than sputtering and enables the economic production of recording media.
The powder of magnetoplumbitic ferrite, for example, Ba-ferrite, comprises particles in a hexagonal tabular form which have an axis of easy magnetization in a direction perpendicular to the plate surface. Therefore, when a slurry of this powder is coated onto a base film, the individual particles are easily oriented parallel to the surface of the base film, providing monoaxial anisotropy in a direction perpendicular to the coated surface. Because of these features, the Ba-ferrite powder is advantageously used as a magnetic material for perpendicular magnetization recording. Also advantageous are powders of magnetoplumbitic Sr-ferrite, Pb-ferrite, Ca-ferrite and combinations thereof.
However, magnetoplumbitic ferrite powders cannot be actually used as magnetic materials for perpendicular magnetization recording unless they satisfy several requirements at the same time. First, the saturation magnetization must be as high as possible; for example, Ba-ferrite should have a degree of saturation magnetization that is as close as possible to its theoretical 72 emu/g. Secondly, the coercive force should be reduced such as to be in the range of 200-2,000 Oe. The crystals of magnetoplumbitic ferrites have a large amount of anisotropy and possess high coercive forces which frequently exceed about 3,000 Oe. However, such high levels of coercive force saturate the head and render high-density recording difficult to achieve. Thirdly, the grain size should be in the range of 0.01 to 0.5 .mu.m in order to ensure effective recording and playback at short wavelengths (.ltoreq.1.0 .mu.m) which are typically used in high-density perpendicular magnetization recording. Fourthly, the grains should be in a thin tabular form and their tabularity ratio (breadth/thickness) is preferably at least 3. In addition to these requirements, a paint having the ferrite particles dispersed uniformly therein is essential for preparing a good recording medium and in order to attain this object, highly dispersible ferrite particles having no sintered or agglomerated grains are necessary.
As is well known, magnetoplumbitic ferrite powders can be produced by either the dry method or the hydrothermal method. They have their own merits and demerits and none of the techniques proposed so far are capable of producing ferrite powders that satisfy all of the requirements listed above.
Japanese Unexamined Published Patent Application No. 125219/1981 discloses producing a magnetoplumbitic ferrite powder for use in perpendicular magnetization recording by the dry method. Japanese Unexamined Published Patent Application Nos. 149328/1981 and 160328/1981 show the use of the hydrothermal method in producing the same magnetoplumbitic ferrite powder.
Stated more specifically, the process shown in Japanese Unexamined Published Patent Application No. 125219/1981 comprises the following steps: mixing predetermined proportions of the basic components of the intended magnetoplumbitic ferrite powder, a component to reduce the coercive force and a glass former; melting the mixture; rapidly quenching the melt to form an amorphous material; subjecting said amorphous material to heat treatment so as to crystallize fine ferrite particles; then removing the glassy material so as to prepare a ferrite powder comprising particles of a size ranging from 0.01 to 0.3 .mu.m and having a coercive force of 200 to 2,000 Oe. This method is quite complicated and unsuitable for use in industrial applications. Furthermore, the expected difficulty in completely removing the glassy material would produce ferrite particles which are not highly dispersible and will cause agglomeration due to the presence of residual glassy material which functions as a binder between individual grains.
The hydrothermal method shown in Japanese Unexamined Published Patent Application No. 149328/1981 is more straightforward; a solution containing metal salts of Fe, Ba having a molar ratio of 1:12-3:12 with respect to Fe, and metallic elements other than Fe and Ba having an average ionic valence of 3 and which are present in molar ratios corresponding to 1.0:12-1.8:12 with respect to Fe is prepared, and after adding an alkali solution in an amount which is one to five times the equivalent weight of the sum of the metal salts, the mixture is subjected to hydrothermal reaction in an autoclaveaat 400.degree. C. or higher. But in this method, which sees high temperature and pressure, sintered Ba-ferrite grains are frequently produced and the desired ferrite powder having good dispersibility is difficult to obtain. Another problem is caused by the fact that a ferrite powder having high saturation magnetization is usually difficult to prepare by autoclaving. In an actual embodiment of the cited process, a reaction was performed for 2 hours in an autoclave maintained at a temperature as high as 550.degree. C., but the resulting Ba-ferrite powder had a saturation magnetization of only 47 emu/g. This value is even lower than 50 emu/g, which is the normal value for the conventional Ba-ferrite used in magnets. Therefore, according to the hydrothermal method shown in Japanese Unexamined Published Patent Application No. 149328/1981, even if the temperature and pressure in the autoclave are elevated to the highest possible values (the autoclave temperature of 550.degree. C. causes an extraordinarily high pressure and cannot be realized without using a highly sophisticated autoclave), the resulting magnetoplumbitic ferrite powder has a saturation magnetization which is far smaller than the theoretical value.
Japanese Unexamined Published Patent Application No. 160328/1981 proposes a two-stage process which is designed to eliminate the disadvantages shown above. According to this process, a Ba-ferrite precursor having a very small saturation magnetization is first prepared by preventing the grain growth and the sintering or agglomeration of grains in an autoclave which is maintained.at low temperatures of 150.degree.-250.degree. C., and then the precursor is heated at 800.degree. C. or higher so as to provide a Ba-ferrite powder having high saturation magnetization. However, even this process is unable to provide the Ba-ferrite powder with good dispersibility since the heating at 800.degree. C. or higher unavoidably causes the sintering or agglomeration of individual grains.
As shown above, the production of magnetoplumbitic ferrite by the conventional dry process unavoidably involves the sintering or agglomeration of ferrite grains, and the sort of paint comprising a uniform dispersion of ferrite particles that is necessary for producing a coated type perpendicular magnetization recording medium adapted to high-density recording cannot be prepared. The dry process is also unable to produce tabular grains having a sufficient tabularity ratio to prepare a coated type perpendicular magnetization recording medium.
The hydrothermal process is capable of producing tabular ferrite grains with a high tabularity ratio but none of the conventional techniques of hydrothermal synthesis have succeeded in attaining the desired saturation magnetization. The Ba-ferrite powder prepared by the method shown in Japanese Unexamined Published Patent Application No. 149328/1981 has a saturation magnetization of 47 emu/g and this would be the highest of the values achieved by the previously reported hydrothermal techniques. This value is still lower than 72 emu/g which is the theoretically possible level for Ba-ferrite powder. In addition, the value of 47 emu/g can only be achieved with the autoclave temperature of 550.degree. C. that calls for industrially unobtainable high temperature and pressures. Even such extreme autoclaving conditions are unable to provide a Be-ferrite powder having a near-theoretical value of saturation magnetization. As a further problem, the higher the temperature and pressure in the autoclave, the greater the possibility that sintered or agglomerated grains are produced. Such grains are not uniformly dispersed in a paint and fail to produce the desired coated type perpendicular magnetization recording medium. As shown in Japanese Unexamined Published Patent Application No. 160328/1981, if the practically feasible autoclaving temperature, for example, 400.degree. C. or lower, is employed, only an incomplete magnetoplumbitic ferrite powder having low saturation magnetization results, and a completely satisfactory product cannot be obtained without supplementing the hydrothermal process with the dry method. But then, the dry method will again introduce the problem of sintered or agglomerated ferrite grains.